Balloon modification for improved stent fixation and deployment

ABSTRACT

An intravascular catheter system and method of manufacture for implanting a radially expandable stent within a body vessel. The catheter system comprises a shaft defining an inflation lumen in fluid communication with a balloon on the distal end of the shaft. A stent is mounted coaxially upon a balloon. The balloon has ridges impressed in the outline of the stent for at least a portion of the stent. The ridges are sized to receive the stent so as to cause the balloon to expand evenly and the stent to deploy uniformly. In an alternative embodiment the balloon may additionally have a polymeric surface layer exhibiting a higher coefficient of friction than the underlying material against the stent material. The surface layer may be provided as a high friction polymeric coating on a formed balloon or as a coaxial extrusion of a high friction polymeric material over the underlying balloon tubing material. In a further embodiment the balloon may have the polymeric surface layer exhibiting a higher coefficient of friction than the underlying material against the stent material but without the ridges.

FIELD OF THE INVENTION

The present invention relates to an intravascular stent deploymentsystem and method of manufacture and more particularly to a catheterballoon for stent delivery with the stent impression being heatset intothe balloon thereby forming ridges to match the stent and/or with a highfriction balloon surface. The ridges and/or high friction surface holdthe stent in place a longitudinally permitting more uniform expansion ofthe stent and reducing the likelihood of the stent slipping along theballoon, inadvertently dislodging or snagging.

BACKGROUND OF THE INVENTION

Percutaneous transluminal coronary angioplasty (PTCA) is used to reducearterial build-up of cholesterol fats or atherosclerotic plaque.Typically a first guidewire of about 0.038 inches in diameter is steeredthrough the vascular system to the site of therapy. A guiding catheter,for example, can then be advanced over the first guidewire. The firstguidewire is then removed. A balloon catheter on a smaller 0.014 inchdiameter guidewire is advanced within the guiding catheter to a pointjust proximal to the stenosis. The second guidewire is advanced into thestenosis, followed by the balloon at the distal end of the catheter. Theballoon is then inflated causing the site of the stenosis to compressinto the arterial wall. The dilatation of the occlusion, however, canform flaps, fissures and dissections which threaten re-closure of thedilated vessel or even perforations in the vessel wall. Implantation ofa metal stent can provide support for such flaps and dissections andthereby prevent reclosure of the vessel or provide a patch repair for aperforated vessel wall until corrective surgery can be performed.Reducing the possibility of restenosis after angioplasty reduces thelikelihood that a secondary angioplasty procedure or a surgical bypassoperation will be necessary.

An implanted prosthesis such as a stent can preclude additionalprocedures and maintain vascular patency by mechanically supportingdilated vessels to prevent vessel collapse. Stents can also be used torepair aneurysms, to support artificial vessels as liners of vessels orto repair dissections. Stents are suited to the treatment of any bodylumen, including the vas deferens, ducts of the gallbladder, prostategland, trachea, bronchus and liver. The body lumens range in size fromthe small coronary vessels to the 28 mm aortic vessel. The inventionapplies to acute and chronic closure or reclosure of body lumens.

A typical stent is a cylindrically shaped wire formed device intended toact as a permanent prosthesis. A stent is deployed in a body lumen froma radially compressed configuration into a radially expandedconfiguration which allows it to contact and support a body lumen. Thestent can be made to be radially self-expanding or expandable by the useof an expansion device. The self expanding stent is made from aresilient springy material while the device expandable stent is madefrom a material which is plastically deformable. A plasticallydeformable stent can be implanted during a single angioplasty procedureby using a balloon catheter bearing a stent which has been crimped ontothe balloon. Stents radially expand as the balloon is inflated, forcingthe stent into contact with the body lumen thereby forming a supportingrelationship with the vessel walls.

The biocompatable metal stent props open blocked coronary arteries,keeping them from reclosing after balloon angioplasty. A balloon ofappropriate size and pressure is first used to open the lesion. Theprocess is repeated with a stent crimped on a balloon. The stent isdeployed when the balloon is inflated. The stent remains as a permanentscaffold after the balloon is withdrawn.

U.S. Pat. No. 4,886,062 to Wiktor for "Intravascular Radially ExpandableStent and Method of Implant" discloses a two-dimensional zig-zag form,typically a sinusoidal form.

U.S. Pat. No. 5,409,495 to Osborn for "Apparatus for UniformlyImplanting a Stent" discloses elastic restraining bands which exert aforce at the proximal and distal ends of the balloon equal and oppositeto that generated by the combined resistance of the sleeve and the stenttending to deform the balloon. In this way, the uneven expansion (endeffects) are limited when the balloon is expanded which, in turn,inhibits a "dog boning" deformation at the proximal and distal regionsof the balloon. FIGS. 3-6 show a balloon of complex manufacture.

European Patent No. 553,960 A1 to Lau for "Protective membrane forStent-carrying Balloon Catheter" discloses a stent mounted on a tubularsheath having an outer surface composed of a high coefficient offriction material designed to secure the stent until balloon inflation.

Copending U.S. Ser. No. 08/637,959 to Rupp et al. discloses a ballooncatheter for stent delivery with the catheter inner lumen tube having agreater outer diameter for a central portion of the area covered by thestent thereby permitting more uniform expansion of the stent.

As stent metal mass increases in stents having elements that can expandindependently in the longitudinal direction, there is a tendency towardslongitudinal compression at the center of the stent when expanded. Theincreased metal mass creates more radial hoop strength which in turnincreases the amount of force required to expand the stent. The centerof the stent has more radial hoop strength than the ends of the stent.As a result, the balloon expands first at the distal and proximal endsbefore expanding at the center. This creates a dumbbell shaped balloon.With the stent ends expanding first, the stent slides down the expandedballoon ends toward the center of the balloon which is as yet unexpandedbecause of the stent's greater radial hoop strength in the center. Whenthe balloon ends have expanded completely, the deployed stent may becompressed to length that is significantly shorter than desired. Becausethe stent is compressed toward the center of the balloon, completeballoon expansion may not be possible. Due to the nature of the PTCAprocedure, as well as handling prior to the procedure, there exists apotential for inadvertent dislodgment of the stent caused by slippage ofthe stent along the deflated balloon.

Bare stenting without a stent sheath presents the additional problem ofthe stent snagging upon luminal calcification. What is needed is amethod of stent deployment which results in uniform stent expansion andreduces the likelihood of the stent slipping along the balloon,inadvertently dislodging or snagging.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a catheter system forimplanting a radially expandable stent which reduces longitudinal stentslippage during stent expansion, reduces snagging during insertion andpermits uniform radial stent expansion. The present invention isaccomplished by providing an intravascular catheter system and method ofmanufacture for implanting a radially expandable stent within a bodyvessel. The catheter system comprises a shaft defining an inflationlumen in fluid communication with a balloon on the distal end of theshaft. A stent is coaxially mounted on the balloon. The balloon hasridges impressed in the outline of the stent for at least a portion ofthe stent. The ridges are sized to receive the stent so a s to reducestent slippage and cause the balloon to expand evenly and the stent todeploy uniformly. In an alternative embodiment the balloon mayadditionally have a polymeric surface layer over an underlying material.The surface layer having a higher coefficient of friction than theunderlying layer. The surface layer may be provided as a high frictionpolymeric coating on a formed balloon or as a coaxial extrusion of ahigh friction polymeric material over the underlying balloon tubingmaterial. In a further embodiment the balloon may have the polymericsurface layer but without the ridges.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevation overall view of a stent fitted over alongitudinal cross-section of a deflated balloon;

FIG. 2 is a side elevation overall view of a stent fitted over aninflated balloon;

FIG. 3 is a side elevation partially cut-away view of an inflatedballoon after the stent has been heat set into the balloon and removed;

FIG. 4 is an enlargement of a portion of FIG. 2;

FIG. 5 is a cross-section of FIG. 1 along the lines 5--5;

FIG. 6 is a cross-section of FIG. 3 along the lines 6--6;

FIG. 7 is a cross-section of a heat chamber with a stent fitted over adeflated balloon therein;

FIG. 8 is a side elevation overall view of a stent fitted over alongitudinal cross-section of a deflated balloon and a crimper with anembedded wire;

FIG. 9 is a side elevation overall view of a stent fitted over alongitudinal cross-section of a deflated balloon with a probe; and

FIG. 10 is a side elevation overall view of a stent fitted over alongitudinal cross-section of a deflated balloon and a crimper with anembedded probe.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Refer to FIG. 1 showing a side elevation overall view of a MedtronicWiktor stent 20 fitted over a longitudinal cross section of a deflatedballoon 35. The Wiktor stent 20 is formed with a wire which is formedinto zig-zags such as a sinusoidal wave form helix pattern the length ofthe stent by a means such as passing the wire through gears such asthose disclosed in U.S. Pat. No. 2,153,936 issued to Owens et al. Thezig-zags are formed by alternate peaks 25 and valleys 30. The zig-zagsare canted toward the proximal and distal ends of the stent 20. Thezig-zags form a plurality of spaced-apart elements 40a-f each extending360 degrees around the hollow cylinder. Each of the elements 40a-f has aplurality of extendible portions such as the zig-zags which permit thewire elements 40a-f to be expanded from an unexpanded diameter as seenin FIG. 1 to a second, expanded diameter as seen in FIG. 2.

A wire having formed zig-zags is wound around a forming mandrel tocreate the cylindrical stent shape. After the stent has been reduced tothe objective outer diameter by compressing it on successively smallermandrels, the proximal and distal ends of the wire segment are attached.The means of attachment may include looping 50 the end segmentstogether, twisting, biocompatible adhesive, brazing, welding orstamping.

For coronary applications, the stent 20 wire can have a diameter ofabout 0.001 inches to about 0.015 inches. The balloon expandable stentcan be made of an inert, biocompatible material with high corrosionresistance that can be plastically deformed at low-moderate stresslevels such as tantalum, the preferred embodiment. Other acceptablematerials include stainless steel, titanium ASTM F63-83 Grade 1, niobiumor high carat gold K 19-22. A self-expanding device can be made by theuse of superelastic nickel titanium (NiTi) such as Nitinol manufacturedby Raychem or Forukawa.

The preferred form of the sinusoidal wave of the stent wire for coronaryapplications is a length of about 0.150 inches to about 0.090 inches anda wave amplitude of between about 0.050 inches and about 0.080 inches.Any wave length and amplitude combination that would provide adequatevessel hoop strength and vessel coverage is appropriate. The stent ofthis invention and balloon can be transported via a standard #7 or 8French guiding catheter. Once on location, the stent can be expandedradially by the expansion of the balloon 35; a ratio of 2.75:1 can beachieved with a wire diameter of approximately 0.005 inches and aninitial stent diameter of approximately 0.060 inches.

As seen in FIG. 1, the stent 20 is centrally located and positioned withrespect to the length of balloon 35. The stent 20 turns are evenlyspaced so that when the stent 20 is expanded the stent 20 will provideeven support inside the vessel and resist external loading. The stent 20must expand evenly and permit the balloon 35 to expand evenly.

The Wiktor stent is formed of a hollow cylindrical wire winding having aplurality of spaced-apart circumferential elements 40a-f. Each element40a-f extends 360 degrees around the hollow cylinder such that theelements 40a-f can move longitudinally as the stent 20 is expanded. Eachof the elements 40a-f has a plurality of extendible portions 25, 30which permit the wire elements 40a-f to be expanded. Each peak 25 andvalley 30 pair comprises a wave. To achieve greater coverage, metal masscan be increased in a sinusoidal wave form stent 20 by having more wavesper revolution, as for example, increasing from four waves to six waves.Stents such as that shown in FIG. 1 having elements 40a-f can expandindependently in the longitudinal direction and can present specialproblems not presented by stents formed of a solid cylinder.

As stent metal mass increases in stents with extendible portions, thereis a tendency towards longitudinal compression at the center of thestent when expanded. The increased metal mass creates more radial hoopstrength which in turn increases the amount of force required to expandthe stent 20. The center of the stent has more radial hoop strength thanthe ends of the stent 20. The balloon expands first at the distal andproximal ends before expanding the center which is covered by the stent.This creates a dumbbell shaped balloon. With the stent ends expandingfirst, the stent slides down the expanded balloon ends toward the centerof the balloon which is as yet unexpanded because of the stent'sincreased radial hoop strength in the center. As the proximal and distalends of the balloon expand to approximately two-thirds of normalexpansion diameter, the mid-section of the balloon begins to expand.When the balloon ends have expanded completely, the stent may have beencompressed to approximately one-half of its original crimped length.This type of balloon inflation seems to act as a moving "snow plow"forcing the stent to slide and contract longitudinally along theballoon. The resulting deployed stent may be tightly bunched, misshapenand significantly shorter than a properly deployed stent. Because thestent is compressed toward the center of the balloon, complete balloonexpansion may not be possible.

To prevent the dumbbell effect as well as reducing the chance of stentdislodgment or snagging, the stent may be embedded into the wrappedballoon and/or the balloon surface may be modified in order to increasethe coefficient of friction to the point that the forces involved in thelongitudinal slippage or inadvertent dislodgment of the stent would notbe able to overcome the friction force. The force required to dislodgethe stent 20 is therefore increased. If a stent dislodges from thecatheter, it could be carried away into the blood stream with seriousconsequences to the patient. Bare stenting without a stent sheathpresents the additional problem of the stent snagging upon luminalcalcification. Recessing 53 the stent reduces stent profile and thelikelihood of snagging.

The stent can be embedded either directly into the balloon as seen inFIG. 3 or into an external polymeric surface layer 110 as seen in FIG.4. This can be done by using a heat chamber 75, a crimper 85/100 or aprobe 95. When the balloon 35 is expanded to deploy the stent 20, theridges 55, 57 created by the heated stent 20 hold the stent 20 in placelongitudinally and allow the stent to expand without significantlyshortening. For any of these methods the balloon 35 must first bewrapped then optionally heatset in any conventional manner.

To wrap the balloon 35 as shown in FIG. 5, the following procedure canbe used. For coronary applications, begin with a 0.0160 inch diameterheat set mandrel. A heat set mandrel is TEFLON® coated as opposed to astainless steal mandrel, for example. TEFLON® does not stick and makesthe mandrel easier to remove. TEFLON®, a form of polytetrafluoroethylene(PTFE) is available from E.I Du Pont de Nemours & Company, Wilmington,Del. Next attach the catheter inflation port to the vacuum manifold.Insert the 10-30 cm mandrel into the distal end of the catheter leavingapproximately 10-20 mm of the mandrel extending from the distal tip ofthe balloon. Deflate the balloon using a vacuum. Fold the balloon sothat it wraps evenly around the guidewire tube in an "S" folding patternas seen in FIG. 5. With vacuum applied and the balloon 35 folded, slidea heat set sleeve over the balloon to hold the balloon in place. Insertthe distal end of the catheter into a heat chamber. A heat chamber 75such as that shown in FIG. 7 can be made in any conventional mater asfor example, by utilizing an insulated chamber with a time andtemperature controller. Apply heat of 75 degrees F. (1 degree C.) for 10minutes under a vacuum of a minimum of 24 inches Hg. Allow the balloonto cool for 5 minutes then remove it from the heat chamber anddisconnect from the vacuum source. The stent 20 can then be crimped uponthe wrapped and heatset balloon 35 either manually or with anappropriate tool.

Once the balloon 35 is wrapped, optionally heatset, and has had thestent 20 crimped about it, the balloon 35 and stent 20 may be placedinto a heater block type of heat chamber 75 such as that shown in FIG.7. The heat chamber 75 can be one or two pieces and may be made ofaluminum. The chamber 75 inside diameter is less than the outsidediameter of the wrapped balloon 35 with stent 20 crimped thereon. Thechamber 75 is in contact with the stent 20 thereby heating the stentwire 20 which becomes slightly embedded into the balloon 35. As seen inFIGS. 3 and 6, this creates ridges 55, 57 in the balloon 35 which matchthe stent 20.

An alternative method for embedding the stent 20 into the balloon 35 canbe used during the final crimping sequence. This procedure utilizes acrimping device as seen in FIGS. 8 or 10 or a probe as seen in FIG. 9.Any electrically conductive stent can be utilized, be it metal orpolymer. For example, tantalum, nitinol or any type of stainless steelis preferred. To embed the electrically conductive stent, a controlledpositive and negative electrical current/charge is applied, in anynumber of locations and orientations to the stent so that the resistanceof the stent causes the stent to heat, thus embedding the stent in thedelivery system. This can occur before, during or after the finalcrimping of the stent. A constant or variable amount of current suitablefor the stent and/or balloon materials selected may be used inconjunction with any amount of radial compression necessary to cause anydegree of embedment desired.

The crimping device can either utilize embedded wires 90 in the crimpingdevice 85 of FIG. 8 or an embedded probe 105 in the crimping device 100of FIG. 10. In FIGS. 8-10 the crimping device 85 and 100 or probe 95comes into direct contact with the stent during the final crimpingsequence. The stent is placed upon a wrapped balloon. The balloon neednot be heat set. In FIGS. 8-10, a controlled amount of electricity isapplied to the stent which will heat the stent and create a greateramount of stent embedding by literally melting the stent into theballoon material. The advantage of utilizing a crimper 85 and 100 orprobe 95 to heat the stent as opposed to a heat chamber 75 is that theballoon is not heated. Localized heating of the areas in which the stentcomes into contact with the balloon is preferable to heating the entireballoon. Heating the entire balloon may cause shrinking.

A suitable temperature for creating the appropriate depth stent 20shaped ridges 55, 57 in the balloon 35 is that which is sufficient toreach the melt indicator for the chosen material. If the stent 20 ismelted directly into the balloon polyethylene layer, use at least 190degrees F., the melt indicator for polyethylene. Use a typicalpolyethylene material in catheter balloons such as Rexene ® Resins 1017or 1011 (available from Rexene of Odessa, Tex.) with a melt index of2.0. The 190 degree F. temperature should be maintained for about 15 to30 seconds under pressure applied by clamp weight. Within these rangesshortening is minimal and within 1 mm. Those skilled in the art wouldrecognize that time and temperature vary inversely such that a highertemperature could be applied for a shorter duration. For example, radiofrequency (RF) heat could be applied for a very short duration. If thetemperature is too high or the time too long, the balloon will notexpand to its proper diameter as the ridges 55 will become too deep.This was the case with a temperature of 210 degrees F. (99 degrees C.)at one-and-one-half minutes to two minutes. Deep ridges 55, 57 also riskperforation and balloon bursting under lower pressures.

The depth of the ridges 55, 57 is a function of the balloon surface andthickness as well as stent wire diameter. The diameter of a typicalstent wire is about 0.005 inches while the wall thickness of a typicalballoon is 0.002 inches. The depth of the ridges 55, 57 should be morethan 5% of the stent 20 wire diameter to reduce slippage and less than50% of the balloon wall thickness to minimize balloon perforations. Theridge 55 depth should preferably range from about 0.00025 inches to0.001 inches.

The balloon 35 may have a polymeric surface layer 110 formed by coatingor formed by coaxial extrusion. The stent 20 may be melted into thepolymeric surface layer 110 or melted into the underlying layer 120 aswell. If the stent 20 is melted into the polymeric surface layer 110 butnot melted into the underlying polyethylene layer 120, conditionsappropriate for deforming the polymeric surface layer 110 while notdeforming the underlying polyethylene layer 120 are used.

In addition to, or instead of, embedding the stent into the balloon, ahigh friction balloon surface 110 can be provided over the underlyinglayer 120 to reduce the likelihood of stent slippage. The underlyinglayer 120 has a first coefficient of friction and the surface layer 110a second coefficient of friction. The second coefficient of friction isgreater than the first coefficient of friction. Such a high frictionsurface layer 110 can be provided by either coating the balloon with ahigh friction polymer carried in an appropriate solvent or by coaxiallyextruding the surface layer 110. The coefficient of friction of thesurface layer 110 against the stent should be high enough to prevent thestent from sliding along the balloon prior to and during the PTCAprocedure, but not so high as to inhibit proper release and deploymentof the stent. If a stent is embedded into a balloon with a high frictionpolymeric surface layer 110, the surface would be provided prior toembedding the stent allowing for adequate adhesion of a consistentlythin high friction film to the balloon.

One method of providing a high friction surface layer 110 is to dip thePTCA balloon into the solvated polymeric coating material, then allowthe solvent to evaporate. If the polymeric surface layer 110 seen inFIG. 4 is provided as a coating, the preferred material should be easilysolvated and compatible with the underlying balloon material. Thepreferred material for the coated surface layer 110 seen in FIG. 4 is ahigh friction elastomeric polymer such as poly(vinyl acetate) with amole weight of 140,000 at 100 mg/ml in acetone (available from AldrichChemicals in Milwaukee, Wis.). Other suitable materials for deliveringthe surface layer 110 as a coating include silicone based polymers suchas NuSil 1081 (available from NuSil Technology located in Carpinteria,Calif.) at 10% in 1:1 ethyl acetate:acetone. Coatings such as poly(vinylacetate) are preferable to silicone based pressure sensitive adhesivesbecause silicone based pressure sensitive adhesives may attract and holdmore foreign matter, e.g., dust and lint, than is desirable.

Another method of providing a high friction surface is to coaxiallyextrude an elastomeric polymer as an outer layer of the material beingextruded as balloon tubing. If the polymeric surface layer 110 isprovided as a coaxial extrusion, the preferred material will becompatible with the underlying balloon material as well as exhibit acomparable melt index. The preferred material for a coaxially extrudedpolymeric surface layer 110 seen in FIG. 4 is a high frictionelastomeric polymer such as poly(ethylene vinyl acetate) with a meltindex of 3.0 (available as Elvax 265 from E.I. Du Pont de Nemours &Company, Wilmington, Del.).

Regardless of the method of providing the polymeric surface layer 110,materials with lower melt temperatures than LDPE, such as poly(vinylacetate) and poly(ethylene vinyl acetate), are preferable because theyallow the stent to become embedded at lower temperatures with lessballoon shrinkage and less potential for damage to the balloon. Polymersexhibiting greater elasticity than LDPE, such as poly(vinyl acetate) andpoly(ethylene vinyl acetate), are preferred since they place lessrestraint on compliant balloons and are less likely to fracture ordelaminate during balloon expansion. The resulting balloon should havean outer surface that has a coefficient of friction high enough toprevent the stent from sliding along the balloon prior to and during thePTCA procedure, but not have so high a coefficient of friction as toinhibit proper release and deployment of the stent.

The thickness of the polymeric surface layer 110 may range from a fewmonolayers for a coating up to 0.002 inches depending on the materialand the method of delivery. The overall inner diameter of a typicalballoon tube is approximately 0.020 inches. The overall outer diameterof the balloon 35 is 0.0405 inches. If a coaxial extrusion of thesurface layer 110 is employed, the wall thickness of the outer layer 110is approximately 0.002 inches while the thickness of the underlying LDPElayer 120 is approximately 0.00825 inches. The surface layer 110 shouldconstitute about 20% of the balloon tube thickness for coaxialextrusions.

The preceding specific embodiments are illustrative of the practice ofthe invention. It is to be understood, however, that other expedientsknown to those skilled in the art or disclosed herein, may be employedwithout departing the scope of the appended claims.

    ______________________________________    No.            Component    ______________________________________    20             Stent    25             Peak    30             Valley    35             Balloon    40a-f          Element    45             Marker Band    50             End Loop    53             Recess    55             First Ridge    57             Second Ridge    60             Guidewire Lumen    65             Inflation Lumen    70             Shaft    75             Heat Chamber    80             Guidewire Tube    85             Crimper with embedded wire    90             Embedded Wire    95             Probe    100            Crimper with Embedded Probe    105            Embedded Probe    110            Polymeric Surface Layer    120            Underlying Layer    ______________________________________

What is claimed is:
 1. An intravascular catheter system for implanting aradially expandable stent within a body vessel the system comprising:acatheter comprising:a shaft defining an inflation lumen having aproximal end and a distal end; an inflatable balloon having a proximalend and a distal end, the balloon proximal end being sealingly affixedto the distal end of the shaft, the balloon being in fluid communicationwith the inflation lumen, the balloon having an outer diameter, theballoon having a surface layer over an underlying layer, the surfacelayer having a lower melt temperature than the underlying layer suchthat heating the surface layer will not damage the underlying layer; astent mounted coaxially upon the balloon, the stent having a thickness,a portion of the stent thickness being recessed into the balloon surfacelayer and forming a permanent first and second ridge deformation forreceiving at least a portion of the stent so as to reduce stent slippageor snagging and cause the balloon to expand evenly and the stent todeploy uniformly, wherein the stent is formed of a helically wound wirehaving longitudinally movable elements, the wire wound into a zig-zagpattern and wherein the first and second ridge define a recess in theouter diameter of the balloon, the recess sized to receive the helicallywound wire.
 2. A catheter system according to claim 1 wherein thezig-zag pattern is canted toward the proximal and distal ends of thestent.
 3. A catheter system according to claim 1 wherein the depth ofthe recess is more than 5% of the stent diameter.
 4. A catheter systemaccording to claim 1 wherein the depth of the recess is less than 50percent of the balloon wall thickness.
 5. A catheter system according toclaim 1 wherein the depth of the recess ranges from approximately0.00025 inches to approximately 0.001 inches.
 6. A catheter systemaccording to claim 1 wherein the balloon surface layer is comprised of apolymeric material, the recess being impressed into the surface layer,the surface layer having a sufficient coefficient of friction so as toreduce stent slippage or snagging and cause the balloon to expand evenlyand the stent to deploy uniformly.
 7. A catheter system according toclaim 6 wherein the recess is impressed into the surface layer as wellas into the underlying layer.
 8. A catheter system according to claim 6wherein the stent is made of tantalum.
 9. A catheter system according toclaim 6 wherein the surface layer is comprised of an elastomericpolymer.
 10. A catheter system according to claim 6 wherein the surfacelayer is a coating.
 11. A catheter system according to claim 6 whereinthe surface layer is comprised of a NuSil 1081 at 10% in 1:1 ethylacetate:acetone.
 12. A catheter system according to claim 6 wherein thesurface layer is comprised of poly(vinyl acetate) with a mole weight of140,000 at 100 mg/ml in acetone.
 13. A catheter system according toclaim 6 wherein the surface layer is formed by a coaxial extrusion. 14.A catheter system according to claim 6 wherein the surface layer iscomprised of poly(ethylene vinyl acetate).
 15. A catheter systemaccording to claim 6 wherein the polymeric surface layer has a lowermelt temperature than Low-Density Polyethylene.
 16. A catheter systemaccording to claim 1 wherein the stent is formed of a round wire.
 17. Anintravascular catheter system for implanting a radially expandable stentwithin a body vessel the system comprising:a catheter comprising:a shaftdefining an inflation lumen having a proximal end and a distal end; aninflatable balloon having a proximal end and a distal end, the balloonproximal end being sealingly affixed to the distal end of the shaft, theballoon being in fluid communication with the inflation lumen; a stentmounted coaxially upon the balloon, the stent is formed of a helicallywound wire having longitudinally movable elements, the balloon having apolymeric surface layer over an underlying layer, the polymeric surfacelayer having a lower melt temperature than the underlying layer suchthat heating the polymeric surface layer will not damage the underlyinglayer, the polymeric surface layer having a permanent first and secondridge defining a recess sized to receive the helically wound wire, theunderlying layer having a first coefficient of friction, the polymericsurface layer having a second coefficient of friction, the secondcoefficient of friction being greater than the first coefficient offriction, the friction between the stent and the polymeric surface layerbeing sufficient to reduce stent slippage and to cause the balloon toexpand evenly and the stent to deploy uniformly.
 18. A catheter systemaccording to claim 17 wherein the stent is made of tantalum.
 19. Acatheter system according to claim 17 wherein the surface layer is madeof an elastomeric polymer.
 20. A catheter system according to claim 17wherein the surface layer is a coating.
 21. A catheter system accordingto claim 17 wherein the surface layer is comprised of NuSil 1081 at 10%in 1:1 ethyl acetate:acetone.
 22. A catheter system according to claim17 wherein the coating is comprised of poly(vinyl acetate) with a moleweight of 140,000 at 100 mg/ml in acetone.
 23. A catheter systemaccording to claim 17 wherein the surface layer is a coaxial extrusion.24. A catheter system according to claim 17 wherein the surface layer iscomprised of poly(ethylene vinyl acetate).
 25. A catheter systemaccording to claim 17 wherein the stent is formed of a round wire.
 26. Acatheter system according to claim 17 wherein the recess is a permanentdeformation.
 27. A catheter system according to claim 17 wherein thepolymeric surface layer has a lower melt temperature than Low-DensityPolyethylene.
 28. An intravascular catheter system for implanting aradially expandable stent within a body vessel the system comprising:acatheter comprising:a shaft defining an inflation lumen having aproximal end and a distal end; an inflatable balloon having a proximalend and a distal end, the balloon proximal end being sealingly affixedto the distal end of the shaft, the balloon being in fluid communicationwith the inflation lumen, the balloon having an outer diameter theballoon having a surface layer over an underlying layer, the surfacelayer having a lower melt temperature than the underlying layer suchthat heating the surface layer will not damage the underlying layer; astent mounted coaxially upon the balloon, the stent having a thickness,a portion of the stent thickness being recessed into the balloon outerdiameter forming a permanent first and second ridge deformation defininga recess for receiving at least a portion of the stent so as to reducestent slippage or snagging and cause the balloon to expand evenly andthe stent to deploy uniformly, wherein the stent is formed of a woundwire having longitudinally movable elements, the wire wound into azig-zag pattern and wherein the recess is sized to receive thelongitudinally movable elements.